1. Field of the Invention
The present invention relates to a data conversion device and a recording/reproduction apparatus which records and reproduces digital data by means of the data conversion device.
1. Description of the Prior Art
Prior art of data conversion processes includes the 8-10 conversion, and a data conversion device based thereon is disclosed, for example, in Japanese Patent Application Laid-Open No. H-1-317280. This conversion device converts given continuous data words (applied data words) in the form of digital signals into channel words in the form of selected channel codes prior to recording the digital data on the information track of a recording medium, with the channel codes having CDS (Codeword Digital Sum) different from each other. FIG. 1 diagrammatically shows a table having a string of three channel words (C1, C2, C3). One string is assigned with 256 (=2.sup.8) data words which represent numbers from 0 to 255 in decimal notation. For each data word I(i), each string includes a channel word Cj(i) where j=1, 2 or 3. These data words include different values of CDS from each other. Therefore, three channel words can be utilized for one data word I(i).
The 8-10 conversion includes second signals having carrier wave of a relatively low frequency related to the recorded information. These are pilot signals which serve as tracking signals giving information on the relative position of the read-out element related to the information track, namely on the track crossing position.
FIG. 2 is a drawing explanatory of ATF (Automatic Track Finding) servo of the conventional 2-frequency pilot system which controls tracking by using the pilot signals. In FIG. 2, numeral 201 denotes a magnetic tape, numeral 202 denotes A track which has plus azimuth with the pilot signal f1 being recorded thereon, numeral 203 denotes B track which has minus azimuth, numeral 204 denotes A track which has plus azimuth with pilot signal f2 being recorded thereon and numeral 205 denotes B head which has minus azimuth.
The B head 205 is a little wider than the B track 203. Signals reproduced with the B head 205 are signals from the B track 203 and cross-talk signals from the A tracks 202, 204 on both sides. When tracking is correctly performed, the cross-talk signals have the same reproduction levels. By using this fact, the ATF servo performs tracking control by passing the reproduced signal through a BPF with pass bandwidth of f1 and a BPF with pass bandwidth of f2, thereby extracting the pilot signals of f1 and f2, and integrating them to adjust the tracking so that the reproduction levels through f1 and f2 become the same.
In 8-10 conversion, in order to insert the pilot signals of f1 and f2 in the data stream of channel words, control signal corresponding to the pilot signal is generated and channel words are selected from the table of FIG. 1 so the mean value of the recorded digital signals changes approximately corresponding to the change of the pilot signals of relatively low frequency.
8-10 conversion of the conventional information conversion apparatus is arranged as described above. The 8-10 conversion has a conversion efficiency of 80%. Although achieving a higher recording density requires forming pilot signals necessary for narrow track recording and increasing the conversion efficiency further, it is difficult to achieve. The circuit for this purpose should also be as small as possible.
Also in the ATF servo, the signals reproduced from the B head 205 include the signals recorded on the B track 203, which become noise components when extracting the pilot signals with the BPF, thereby resulting in deteriorated S/N ratio of the pilot signals.